1. Field of the Invention
This invention relates generally to the field of particulate matter collection from discharge gases and more particularly to a multi-stage collector that collects both electrostatically and with barrier filters.
2. Description of Related Art
It is well known in the art how to build and use electro-static precipitators. It is also known how to build and use a barrier filter such as a baghouse. Further, it is known how to charge particles so that these charged particles may be collected in a barrier filter with lower pressure drop and emissions than uncharged particles collected at the same filtration velocity.
Prior art designs have been discussed in the U.S. Pat. No. 5,547,493 (Krigmont), U.S. Pat. No. 5,938,818 (Miller), U.S. Pat. No. 5,158,580 (Chang), and U.S. Pat. No. 5,024,681 (Chang). Krigmont teaches a new precipitator electrode design/configuration, while the Miller and Chang deal with a combination of a precipitator or electrostatic augmentation and a barrier filter (fabric filter or a baghouse).
An electrostatic precipitator or collector typically consists of two zones: 1) a charging zone where the dust or aerosol particles are charged, usually by passing through a corona discharge, and 2) a collecting zone where the charged particles are separated and transferred from the gas stream to a collecting electrode with subsequent transfer into collecting or receiving hoppers/bins.
The arrangement of these zones has led to two typical prior art precipitator design concepts: a conventional electrostatic precipitator where both zones are combined in a single-stage, and a so-called two-stage design where the zones are separated.
Particulate matter (which may be waste or may be re-usable) found in waste gases from industry and power plants (hereinafter called by the generic term “dust”), can have various electrical resistance depending on temperature, humidity and other environmental factors. In particular, the resistance of fly ash depends on gas temperature, gas composition (especially moisture and sulfur trioxide), as well as various other coal or ash properties. Resistance is the result of a combination of surface and volume resistivity. Dust is considered to have high resistance when the particulate resistivity is over about 1011 ohm-cm. Dust is considered to have a low resistance when the particulate resistivity is lower than about 104 ohm-cm.
The electrostatic precipitation process, in the case of high-resistance dusts, results in some reverse ionization at the side of the collecting electrode at which the dust accumulates. As a result, positively charged dust particles may be released or formed by such reverse ionization, and naturally such positively charged particles are repelled from, and not attracted to, the positively charged dust-collecting surface. As the gas stream passes between the “conventional” dust-collecting electrodes, particles which pick up a positive charge by reverse ionization near to a collecting electrode tend to move toward the next discharge electrode where they may pick up a negative charge. They may then move toward the collecting electrode where they may again pick up a positive charge, etc. The result is a zigzag motion where the particles are not collected.
In the case of low resistance dust, a somewhat similar process takes place due to entirely different phenomena. Low resistance dusts are known for a quick discharging; thus they would be repelled back into the gas stream almost instantly upon contacting the collecting plates, irrespective of their polarity.
Viewed as a statistical phenomenon, particles of dust tend to move in a zigzag fashion between the plane of the discharge electrodes and the collecting electrodes spaced from them as the gas entrains such particles along the collecting path. The zigzag movement is a phenomenon which is associated with both high and low resistance dusts.
Because of the zigzag phenomenon, the effectiveness of dust collection is reduced, and the performance of a dust-collecting or dust-arresting assembly will be substantially lower for high or low resistance dusts than with dust with a the normal resistance range (particulate resistivity between 104 and 1011 ohm-cm).
Krigmont in U.S. Pat. No. 5,547,493 describes an electrostatic precipitator which utilizes a unique electrode design that provides for separate zones for aerosol particles charging and collection. The dust collecting assembly is a system of bipolar charged surfaces that are constructed in such a way that they provide alternate separate zones for high-voltage non-uniform and uniform electrostatic fields. The surfaces of the electrodes allow combining the charging and collecting zones with non-uniform and uniform electric fields respectively in one common dust arresting assembly. The disadvantage of this design is that it is entirely electrostatic allowing some of the particulate matter to make it past all the electrodes without being collected, especially in the case of high and/or low resistance dust.
Barrier filters (known as baghouse filters) are an alternative to electrostatic collection. They are generally bags through which the gas is made to pass. Conventional designs can be categorized as low-ratio baghouses (reverse-gas, sonic—assisted reverse-gas, and shake-deflate) which generally operate at filtration velocities of 0.76 to 1.27 centimeters per second (1.5 to 2.5 ft/min), also defined as air-to-cloth ratio or volumetric flow rate of flue gas per unit of effective filter area (cubic feet of flue gas flow/min/square foot of filtering area), and high-ratio pulse-jet baghouses which generally operate at 1.52 to 2.54 centimeters per second (3 to 5 ft/min). Baghouses generally have very high collection efficiencies (greater than 99.9%) independent of fly ash properties. However, because of their low filtration velocities, they are large, require significant space, are costly to build, and unattractive as replacements for existing precipitators. Reducing their size by increasing the filtration velocity across the filter bags results in unacceptably high pressure drops and outlet particulate emissions. There is also potential for “blinding” the filter bags, a condition where particles are embedded deep within the filter and reduce flow drastically.
In a barrier filter, the particulate dust is collected on the outside surfaces of the bags while the flue gas passes through the bag fabric to the inside where it exits through the top or bottom of the bags into a clean air plenum and subsequently out the stack. Cages are installed inside the bags to prevent them from collapsing during the normal filtration process. In pulsejet filters air nozzles are installed above each bag to clean the bag. By applying a quick burst of high-pressure air directed inside the bags, the bags are cleaned. This burst of air causes a rapid expansion of the bag and momentarily reverses the direction of gas flow through the bag which helps to clean the dust off the bags.
Because of the small bag spacing and forward filtration through the two rows of bags adjacent to the row being cleaned, much of the dust that is removed from one row of bags is simply recollected on the adjacent rows of bags. Thus, only the very large agglomerates of dust reach the hopper after the burst of air through the bags. This phenomenon of redispersion and collection of dust after bag cleaning is a major obstacle to operating prior art baghouses at higher filtration velocities.
What is badly needed is a particulate collection system that has the high collection efficiency of a barrier filter along with the high filtering velocity of an electrostatic precipitator.